These treatment devices are notably known as a device for heat treatment of a reactive medium, where by <<Meat treatment>> are meant various treatments carried out by heating such as evaporation, drying, roasting, extraction of natural products in suspension in a solvent, reaction or chemical synthesis with heating by dielectric losses (with view of analyzing or producing chemical compounds), dehydration, baking, discoloration, dissociation, polymerization, cross-linking, treatments with supercritical fluids, removal of volatile compounds, etc., as well as various treatments performed simultaneously with heating, such as mixing or milling.
For such heat treatments, it is notably known how to use electromagnetic radiations of the microwave or high frequency type. Microwave electromagnetic radiation relates to waves, the frequency of which is comprised between about 300 MHz and about 30 GHz, preferentially between 400 MHz and 10 GHz, and preferentially between 915 MHz and 2.45 GHz. High frequency electromagnetic radiation relates to waves, the frequency of which is comprised between about 100 kHz and about 300 MHz, preferentially between 13 MHz and 28 GHz.
Such treatment devices find applications for many types of reactive media, which involve a single reagent or a mixture of reagents in variable proportions, in the solid, liquid or gas state, with or without catalysts, said medium comprising at least one component sensitive to radiation. The reactive medium may be of the solid type (for example of granular or powdery type), of the gas, plasma, liquid type (with a solvent and/or solutes absorbing electromagnetic radiation).
In a known way, a device for treating a reactive medium with electromagnetic radiation comprises:                an electromagnetic radiation generator like a high frequency wave generator or a microwave generator for example in a non-limiting way consisting of a magnetron generator or a semiconductor generator;        a reactor containing said reactive medium, wherein the reactor may assume the shape of a reservoir with optionally means for circulating said reactive medium; and        a device for transmitting the electromagnetic radiation generated by the generator to the reactive medium contained in said reactor.        
This transmission device conventionally comprises:                means for transmitting electromagnetic radiation positioned at the output of the generator and coupled with said reactor in order to transmit the electromagnetic energy to the reactor, these transmission means being of course adapted to electromagnetic radiation and which may for example consist of a transmission cable, such as a coaxial cable, or a wave guide conventionally used in the field of microwaves;        coupling means arranged for allowing transfer into the reactive medium of the electromagnetic energy from the transmission means.        
When operating, the generator generates electromagnetic radiation at a given frequency, the transmission device transmits the generated electromagnetic radiation to the reactive medium via the transmission means and the coupling means.
The coupling means generally comprise a device for applying the energy to the reactive medium, currently called an energy applicator, the selection of which depends on the radiation means (high frequencies and microwaves), on dimensional characteristics of the medium to be treated and on its treatment method.
For high frequency applicators, the following applicators are notably known:                capacitive applicators formed with two capacitor plates between which the high frequency voltage is applied;        inductive applicators for treating sufficiently conductive materials, these applicators consist of a solenoid energized with a high frequency current;        applicators with alternating bars for relatively planar materials consisting of tubular or bar electrodes;        applicators with alternating rings or loops for thread-like materials forming the electrodes.        
The major drawback of these applicators is that they are not very or not adapted for ensuring energy transfer in a liquid mass and homogeneously.
For microwave applicators, the following applicators are notably known:                applicators with a localized field of the single-mode cavity type;        applicators with a diffuse field of the multi-mode cavity type;        applicators with a near field of the radiating antenna guide type.        
As regards applicators with a localized field or with a diffuse field, they require a reactor at least partly consisting of a material transparent to the waves, i.e. not absorbing the waves, such as for example polytetrafluoroethylene or quartz, which is positioned inside the cavity of the applicator. The reactor containing the reactive medium is then subject to electromagnetic radiation coming from the outside.
The applicator with a localized field, of the single-mode type, is formed with a single-mode cavity of predetermined size, resonating at the emission frequency according to radiation in the direction of the waveguide. This single-mode cavity allows a relatively homogeneous distribution of the electromagnetic field inside the cavity. Nevertheless, with this type of single-mode applicator, the amount of material to be treated is limited by the dimensions of the cavity and therefore of the waveguide. For an industrial application, it is necessary to provide a complex and costly apparatus comprising several single-mode applicators placed in parallel in order to have sufficient output, as well as a complex circulation system for the reactive medium. Further, the transferable electromagnetic energy is limited by the volume at the interface between the product to be treated and the radiation.
The applicator with a diffuse field, of the multi-mode type, on the other hand provides a non-homogeneous distribution of the electromagnetic field inside the cavity, with the presence of hot points. Such a distribution limits the volume of the sample to be treated in the applicators of the multi-mode type, and furthermore requires setting into motion or stirring of the sample in order to ensure homogeneity of the heating by microwaves.
A common drawback to both of these applicators of the single-mode type or multi-mode type is that they require reactors in a suitable material in order not to absorb the waves. In addition to being particularly complex and costly to produce, these reactors transparent to the waves are limited in size and in shape, thereby limiting the treatment by waves to certain reactions and certain reactive media, and excluding other reactions where the shape and the length of the reactor may have a predominant role. It is also noted that for reactions which have to be conducted under high pressure, independently or not of the heating induced by the electromagnetic radiation, these wave-transparent reactors most often withstand high pressures with difficulty.
The other drawbacks common to both of these applicators, of the single-mode or multi-mode type, result from the difficulty of obtaining a constant electric field in the reactor, and finally from the fact that the geometry of the cavity depends on the radiation frequency so that an applicator with a localized field can only operate at a given single frequency.
A last drawback of both of these applicators, of the single-mode or multi-mode type, is that they are not compatible with a device for accurately controlling the heating temperature of the reactive medium by the use of an outer coolant fluid. Indeed, in order to accurately and optimally control a reaction, it is particularly advantageous that a liquid coolant circulates around the reactor in the heating area. If this was the case, the waves would heat the liquid coolant, which clearly cannot be considered. If the liquid coolant circulated in a conduit non-transparent to the waves, the conduit would in this case form a screen between the electromagnetic radiation and the reactor, clearly limiting the exposure of the reactive medium to the radiation and therefore the yield of the heating. These applicators therefore prevent temperature control in the irradiation area, i.e. in the cavity, so that control of temperature is sometimes inefficient or even incompatible with certain chemical reactions. This is notably the case when it is necessary to maintain a low temperature during the treatment of materials of biological origins which may thermally degrade after irradiation with electromagnetic waves.
As regards the applicators with a near field of the radiating antenna guide type, they are known from the prior art notably from European patent application No. EP 0 329 338 which discloses a device for treating by microwaves ceramic powders in a high pressure environment. This device includes a reactor forming a high pressure resonant cavity, in the interior of which microwave radiation is introduced by a rectilinear antenna which partly extends into the cavity.
An applicator with a near field is also described in document DE 10 2004 057 851 A1, wherein a rectilinear antenna of the straight antenna or stick antenna type, is directly introduced into the interior of a reactor, containing a gas, liquid or solid medium. A coaxial cable, comprising an internal conductor and an external conductor, is coupled with the reactor in the following way: the external conductor is coupled with the external wall of the reactor and the internal conductor opens out into the reactor at the coaxial cable/reactor junction. The end portion of the internal conductor thus forms an antenna which radiates into the reactive medium which heats up by dielectric loss. The length of the antenna is greater than the wavelength of the radiation.
International application WO 00/15008 describes an applicator with a near field having a radiating antenna. This device comprises a cable, one first end of which is connected to a receiver for receiving the waves generated by a wave generator, and one second end of which cooperates with a transmitter for transmitting the waves into the reactor. The cable may be of the coaxial type and the transmitter corresponds to an exposed portion of an internal conductor of said coaxial cable. This exposed portion of the internal conductor has a length of about one quarter of the radiation wavelength, and it plays the role of a radiating rectilinear antenna inside the reactor.
International application WO 2006/067579 describes an applicator with a near field similar to the one described in international application WO 00/15008, with a radiating antenna. This antenna at its free end has one or more exposed portions of an internal conductor which radiates inside a reactor, the effect of the radiation is thus focused on the end of the antenna. In order to avoid the effects of too localized radiation and to densify the distribution of the energy, in this document, the use of two or more antennas which emit waves in phase according to the principle of interference of coherent electromagnetic waves is contemplated. The application of this principle involves the use of expensive techniques, with several antennas coupled with the wave generator and with a device for controlling the in-phase energizing of the antennas from a same wave generator.
Patent application US 2006/001563 A1 describes a device used in separating a mixture, such as for example a liquid mixture or a colloidal suspension, wherein the device comprises a rotor assembly interiorly divided into one or more chambers. This document discloses the principle of using at least one microwave generator for transmitting microwave energy into the chamber(s). In a first configuration, the microwave generators are positioned inside an internal tubular shaft extending inside the rotor assembly wherein at least one portion of the internal tubular shaft is substantially transparent to microwave energy. This document does not detail the structure of the microwave generator and further does not give any teaching for one skilled in the art on the application of these microwave-generating means within this separation device, except for a vague indication on the use of an antenna without any further specification as to the selection and use of such an antenna. In a second configuration, the microwave generator(s) comprise(s) one or more wave guides formed inside the internal tubular shaft in order to emit microwave radiation into the chamber(s) through transparent portions of said shaft; the use of such wave guides not operating for generating microwaves inside a reactor.
Radiating antennas solve part of the drawbacks of the applicators of the single-mode or multi-mode type, notably because they allow the radiation to be directly transmitted towards the interior of a reactor, thereby avoiding the use of a reactor transparent to the waves with all the constraints mentioned above.
Nevertheless, these applicators with radiating rectilinear antennas have a certain number of drawbacks. Firstly, the electromagnetic energy transmitted to the medium and the frequencies which may be used are limited by the actual properties of the antenna, i.e. in that the attenuation of the wave in an antenna depends on the frequency of the radiation and on dielectric characteristics of the reactive medium.
One of the main drawbacks of rectilinear antennas is the small length of the radiating portion which limits the energy which may be transferred by such a device and entails increased risks of dielectric breakdown at the interface between the antenna and the reactive medium, the consequence being the formation of quite undesirable electric arcs in the case of the treatment of flammable or explosive media.
Further, a heat treatment device provided with a radiating rectilinear antenna is limited to a given single frequency, depending on the dimensions of the antenna, and is limited to non-conductive dielectric reactive media having neither too great nor too low absorption. Conversely, the length of the antenna is itself limited by the frequency of the radiation and by the reactive medium. With such a radiating antenna, if the frequency of the radiation increases, the length of radiant antenna actually in the medium also decreases and also the electromagnetic energy transmitted to the medium also decreases.
Such a device cannot ensure a constant electric field in the medium.
In a known way, an antenna is a device for radiating electromagnetic waves. Such antennas never radiate in a uniform way, i.e. in the same way over the total length of the antenna. In fact, the energy radiated by an antenna is unevenly distributed in space or in a confined medium.
Thus, between the portion where the antenna is exposed to the reactive medium, said base of the antenna, and its free end, radiation is attenuated, in other words the radiation diagram is weakened along the antenna. This attenuation is illustrated in FIGS. 1a and 1b wherein a rectilinear radiating antenna respectively A1 and A2, is immersed in a reactive medium M. The radiating antenna A1 illustrated in FIG. 1a extends over its whole length in the reactor R, here consisting of a container filled with the reactive medium M, while the radiating antenna A2 illustrated in FIG. 1b corresponds to an end portion of an internal conductor Cin of a coaxial cable C which has been stripped; the coaxial cable comprises an external conductor Cout, a dielectric layer Cd and the internal conductor Cin, wherein the external conductor Cout and the dielectric layer Cd have been removed in order to expose a length portion of the internal conductor Cin which thus forms the antenna A2.
The antennas A1 and A2 radiate electromagnetic energy into the medium M, the radiation diagram bearing reference D in the figures. The propagation velocity and the attenuation of the wave depend on the dielectric characteristics of the medium. The attenuation of the wave may vary from a factor of one to a thousand depending on the reaction media, for example water will attenuate the wave about a thousand times more than a very weakly polar medium such as for example hexane.
In practice, the dielectric constants may be caused to vary during a chemical reaction, as the reagents gradually transform while adopting dielectric characteristics which change constantly.
Depending on the medium, the effective radiating length LR of the antenna is more or less long from the point where the antenna is exposed to the medium, so that the radiation is very localized. In a sufficiently homogeneous medium, such as for example molten glass, or in a not very absorbent medium, the effective radiating length may substantially correspond to the total length of the antenna. On the other hand, in other media, attenuation of the radiation occurs relatively rapidly along the antenna, thereby corresponding to a very short effective radiating length LR, in certain cases limited to a few centimeters.
Thus, such antennas have the drawback that the heating by dielectric loss is more or less homogeneous in the medium. Further, the heating is more or less well distributed along the antenna during a reaction wherein the dielectric constants change over time.
Further, these antennas generally require shielding against the radiation, in particular when the reactor is of the wave-transparent type, such as for example in glass or in Pyrex conventionally used in chemistry laboratories.